Typically, optical amplifiers widely used in an optical communication system have different gains with respect to different channels (i.e., different wavelengths). Provided that one channel among a plurality of channels has a relatively low gain or a relatively high gain as compared to the remaining channels, there may occur a serious problem in such optical amplifiers. In more detail, in the case where signals are attenuated or repeatedly re-amplified in an optical amplification system, such difference in gain between different channels can accumulate continuously, and thus increase to levels capable of damaging the optical amplification system. Therefore, gain flattening is needed to give different channels (i.e., different wavelengths) the same gains.
Conventionally, in order to achieve the gain flattening, there has been widely used either a method for modulating the composition of a gain medium making up the optical amplifier, or a fixed method using a gain flattening filter. However, gain characteristics of optical amplifiers vary with peripheral environments of the optical amplifiers, such that it is difficult for the fixed method to effectively cope with such variable gain characteristics of the optical amplifiers.
In order to solve this problem, there has been proposed a dynamic gain equalizer for variably equalizing the output of optical channels. The dynamic gain equalizer includes an optical branching filter (also called an optical demultiplexer) for dividing a multiplexed light source into signals of individual wavelengths, and thus generating divided optical power signals; an attenuator array for attenuating the divided optical power signals according to their channels; and an optical multiplexer for collecting channel signals equalized by an attenuator. The optical multiplexer is symmetrical to the optical branching filter functioning as the optical demultiplexer. There have been proposed a variety of modified dynamic gain equalizers using such symmetrical characteristics between the optical multiplexer and the optical demultiplexer, for example, a method for arranging a reflector at the symmetric center between them, and a method for inducing optical attenuation by inducing variations in optical phase differences, etc. Although there are a variety of modified dynamic gain equalizers, they have the same principles and are based on optical attenuation technologies. Because the gain equalizers are based on the optical attenuation technologies, they are respectively called an attenuated gain equalizer (i.e., a loss-type gain equalizer). The loss-type gain equalizer can be implemented with various methods, for example, either a construction method of the optical multiplexer and the optical demultiplexer for use with a bulk grating, an arrayed waveguide grating (AWG) or a dielectric filter, etc., or an attenuation method used for a phase interference process or an attenuator array, etc. However, the signal attenuation causes the SNR (Signal to Noise Ratio) to deteriorate in general communication systems, and therefore it is necessary for a system to equalize optical channel output or gain on the condition that there is no signal attenuation.